The entire disclosure of Japanese Patent Application No. 2000-390936 filed on Dec. 22, 2000 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a method for forming a magnetic pattern in a magnetic recording medium such as a magnetic disk used for a magnetic recording device, or a photomask used for such method. Also, the present invention relates to a magnetic recording medium produced by using the photomask or a magnetic recording device.
2. Discussion of Background
Magnetic recording devices represented by a magnetic disk device (a hard disk drive) have widely been used as external memory devices for information processing devices such as computers, and have recently been used as recording devices such as devices for recording dynamic images or for set-top boxes.
A typical magnetic disk device comprises a shaft for holding a single or plurality of magnetic disks by penetrating the center of the magnetic disk or disks, a motor for rotating the magnetic disk or disks which is or are connected to the shaft by interposing a bearing or bearings, a magnetic head for recording/reproducing information, an arm for supporting the magnetic head and an actuator for moving the magnetic head via the arm to a desired position on the magnetic recording medium.
As the magnetic head for recording/reproducing signals, a flying head capable of moving above the magnetic disk at a constant flying height is generally used. Other than the flying head, a contact head is proposed in order to make the distance to the medium closer.
A magnetic recording medium (a magnetic disk) to be placed on the magnetic disk device is prepared generally by forming a NiP layer on the surface of a substrate comprising an aluminum alloy, applying a predetermined smoothing treatment, a texturing treatment or the like thereon, and then, forming successively a metallic underlayer, a magnetic layer (an information recording layer), a protective layer, a lubricant layer and the like in this order thereon. Or, it may be prepared by forming successively a metallic underlayer, a magnetic layer (an information recording layer), a protective layer, a lubricant layer and the like on the surface of a substrate made of glass or the like.
The magnetic recording medium includes a longitudinal magnetic recording medium and a perpendicular magnetic recording medium. In the longitudinal magnetic recording medium, longitudinally recording is generally conducted.
The speed of increasing the magnetic density of magnetic recording media is more year by year, and various techniques for increasing the density have been proposed. For example, there are attempts to make the flying height of the magnetic head smaller, to employ a GMR head as the magnetic head, to improve a magnetic material used for the recording layer of the magnetic disk so as to have a strong coercive force, and to reduce the space between tracks for recording information of the magnetic disk. For example, a density of track of 100 Ktpi or more is needed in order to realize 100 Gbit/inch2.
In each track, a magnetic pattern for controlling the magnetic head is formed. For example, it produces signals used for controlling the position of the magnetic head or signals used for synchronous control. When the space between adjacent information recording tracks is narrowed to increase the number of tracks to thereby obtain a high recording density, it is necessary to make signals for controlling the position of a data-recording/reproducing head (hereinbelow, referred to as xe2x80x9ca servo signalxe2x80x9d) dense in a radial direction of the disk in response to the increased number of tracks, i.e., to generate the signals much more so that a precise control can be performed.
On the other hand, it is necessary for the high recording density to increase the data recording capacity by reducing the surface area other than the area used for recording data, namely, an area used for the servo signals and gap portions between the servo areas and the data recording areas whereby the data recording area can be broadened. For this purpose, it is necessary to increase the output of the servo signals or to increase the accuracy of synchronizing signals.
As a conventional method used widely for manufacturing magnetic recording media, an opening was formed in the vicinity of the head actuator of the drive (magnetic recording device), a pin with an encoder was inserted into the opening to engage the actuator with the pin whereby servo signals were recorded by moving the head to a correct position. However, such method encountered difficulty in recording correctly the servo signals because the position of the gravity center of the actuator was different from the position of the gravity center of a positioning mechanism, so that highly accurate track position control could not be obtained.
On the other hand, there is a proposed technique that laser beams are irradiated to a magnetic disk to deform locally the surface of the disk whereby minute projections and recesses are physically formed so that servo signals are produced by the minute projections and recesses. In this technique, however, there were such problems that the formed projections and recesses made the flying magnetic head unstable to affect adversely recording or reproducing of information; laser beams having a large power was needed for forming the projections and recesses, thus being costly, and it took much time to form the projections and recesses one by one.
In view of the above, some servo signal forming methods have recently been proposed.
As an example, there is a method that a servo pattern is formed in a master disk having a magnetic layer of high coercive force, and the master disk is brought to close contact with a magnetic recording medium and then, an auxiliary magnetic field is applied to the medium from the outside whereby a magnetic pattern is printed (U.S. Pat. No. 5,991,104).
As another example, there is a method that a medium is previously magnetized along a certain direction, a soft magnetic layer of high permeability and low coercive force is formed by patterning on a master disk, and the master disk is brought to close contact with the medium and then, an external magnetic field is applied to the opposite direction of the previous magnetized direction. In this method, the soft magnetic layer functions as a shield, and a magnetic pattern is printed in an unshielded area (see, JP-A-50-60212 (U.S. Pat. No. 3,869,711), JP-A-10-40544 (EP915456), and Digest of InterMag 2000, GP-06). In the above-mentioned techniques, a master disk is used and a magnetic pattern is formed in the medium by applying a strong magnetic field.
The intensity of a magnetic field generally depends on distances. Accordingly, when a magnetic pattern is recorded by applying a magnetic field, the boundary of a formed magnetic pattern is apt to be unclear due to a leaking magnetic field. Accordingly, it is essential to bring the master disk into close contact with the medium in order to minimize the influence of the leaking magnetic field. As the magnetic pattern is finer, it is necessary to bring them to close contact without any gap. Usually, the both members are press-contacted by using vacuum suction. Further, the higher the coercive force of the medium is, the larger the magnetic field used for the printing is, and accordingly, the leaking magnetic field becomes large. Therefore, perfect close contact is desirable.
The above-mentioned techniques are easily applicable to a magnetic disk having a low coercive force or a flexible floppy disk being easy for press contact. However, it is very difficult for these techniques to apply a magnetic disk for high density recording comprising a hard substrate which has a coercive force of 3,000 Oe or more. Namely, in the magnetic disk comprising a hard substrate, there was possibility that fine dust deposits thereon at the time of bringing the disk into close contact with the master disk, whereby a defect was resulted in the medium, or the expensive master disk is damaged. In a case of using a glass substrate, in particular, there was a problem that the deposition of dust may cause insufficient close contact, so that it might be impossible to conduct magnetic printing, or a crack was resulted in the magnetic recording medium.
Further, in the technique described in JP-A-50-60212, there was such problem that a pattern having an angle oblique to a direction of tracks in a disk, although recording is possible, was limited to a pattern which was weak in signal intensity. Namely, in a magnetic recording medium having a high coercive force of 2,000-2,500 Oe or more, it is indispensable, for a ferromagnetic material (for a shielding material) for forming a pattern in the master disk, to use permalloy or a soft magnetic material having a large saturation magnetic flux density such as sendust in order to assure a sufficient magnetic field intensity for printing.
However, in the case of the oblique pattern, a magnetic field of reversed magnetization was oriented in a direction perpendicular to the gap produced by the ferromagnetic layer of the master disk, and it was impossible to incline the magnetization in a desired direction. As a result, a part of the magnetic field escapes into the ferromagnetic layer and a sufficient magnetic field could not be applied to a desired position in magnetic printing whereby a sufficient pattern of reversed magnetization could not be obtained, and it was difficult to produce signals of high intensity. In using the oblique magnetic pattern, the reduction of the output of reproducing signals was larger than the azimuth loss, in comparison with a case of using a magnetic pattern perpendicular to the tracks.
Japanese Patent Application Nos. 2000-134608 and 2000-134611 describe a technique of forming a magnetic pattern in a magnetic recording medium by combining the heating of a local portion and the application of an external magnetic field. Specifically, the method is such that the medium is previously magnetized in a direction, energy beams or the like are irradiated through a patterned photomask to heat a local portion of the medium to lower the coercive force of the heated area, and at the same time, an external magnetic field is applied thereto whereby recording is effected to the heated area, so that a magnetic pattern is formed.
In this technique, it is unnecessary that the intensity of the external magnetic field is higher than the coercive force of the medium since the external magnetic field is applied after the coercive force is lowered by heating, and accordingly, it is possible to form the magnetic pattern with a weaker magnetic field. Further, since the area subjected to recording is limited to the heated area, i.e., the recording can not be effected even when the magnetic field is applied to an area other than the heated area, a clear magnetic pattern can be recorded without bringing the mask to close contact with the medium. Therefore, there is no danger of damaging the medium or the mask by pressing the mask, or there is no possible defect in the medium.
Further, according to this technique, since it is unnecessary to shield the external magnetic field with use of a soft magnetic material for a master disk as required in the conventional technique, an oblique magnetic pattern can be formed well.
The photomask used for the above-mentioned magnetic pattern forming methods can be any as far as it is provided with a transmitting portion and a non-transmitting portion by which a predetermined magnetic pattern is formed. For example, a transparent substrate such as quartz glass, soda lime glass or the like may be used. A metallic layer of Cr or the like is formed on the substrate by sputtering; a photoresist is coated on the metallic layer, and etching or the like is conducted to thereby form the transmitting portion and the non-transmitting portion in predetermined portions. In this case, the portion having the Cr layer on the substrate constitutes the non-transmitting portion and the portion without having any layer on the substrate constitutes the transmitting portion.
As described above, the magnetic pattern forming methods described in Japanese Patent Application Nos. 2000-134608 and 2000-134611 are excellent in forming effectively and accurately a various fine magnetic patterns without damaging the magnetic recording medium or the mask and without increasing defects in the medium. This technique had, however, a problem that an interference fringe might result in the pattern forming surface when the photomask was used, by the reason described below, whereby the accuracy of the magnetic pattern was decreased.
Namely, when light which has transmitted through the photomask reaches the magnetic recording medium, the almost amount of the light is absorbed in the surface of the medium. However, a part of the light is reflected. The reflected light reaches again the mask, and a part of the reflected light is reflected on the surface of the mask whereby an interference fringe is formed. When the interference fringe is formed, a tint pattern of energy beam which is different from the mask pattern is produced in the energy beams whereby the modulation of output signals from the magnetic pattern is deteriorated.
It is an object of the present invention to provide a method for forming a magnetic pattern in a magnetic recording medium, which is capable of forming effectively and accurately a fine magnetic pattern by combining the heating of a local portion and the application of an external magnetic field, and a photomask used for such method.
It is an object of the present invention to provide a magnetic recording medium and a magnetic recording device capable of effecting further high density recording, which can be provided in a short time and economically by utilizing the above-mentioned method and/or the photomask.
According to the present invention, there is provided a method for forming a magnetic pattern in a magnetic recording medium, comprising a step of irradiating energy beams to a magnetic recording medium having a magnetic layer on a substrate via a photomask having a transmitting portion of energy beam and a non-transmitting portion of energy beam to heat locally an irradiated portion of the magnetic layer, and a step of applying an external magnetic field to the magnetic layer, the method being characterized in that the transmitting portion and the non-transmitting portion of the photomask have each a reflectivity of energy beam of 30% or less in at least its one surface facing the magnetic recording medium.
According to the present invention, there is provided a photomask used for a method for forming a magnetic pattern in a magnetic recording medium, including a step of irradiating energy beams to a magnetic recording medium having a magnetic layer on a substrate via a photomask having a transmitting portion of energy beam and a non-transmitting portion of energy beam to heat locally an irradiated portion of the magnetic layer, and a step of applying an external magnetic field to the magnetic layer, the photomask being characterized in that the transmitting portion and the non-transmitting portion of the photomask have each a reflectivity of energy beam of 30% or less in at least its one surface facing the magnetic recording medium.
According to the magnetic pattern forming method, it is unnecessary to use a strong external magnetic field as in the conventional technique because the locally heating and the application of an external magnetic field are used in combination in forming a magnetic pattern. Further, even when the magnetic field is applied to an area other than the heated area, the area applied with the magnetic field is not magnetized, and accordingly, the area for forming magnetic domains can be limited to the heated area. Accordingly, the boundary of the magnetic domains is clear whereby a pattern having a small magnetic transition width, a very steep magnetic transition at the boundary of the magnetic domains and a high quality of output signals, can be formed. Further, the magnetic transition width can be formed to be 1 xcexcm or less by selecting suitably conditions.
Further, it is unnecessary to press-contact the magnetic recording medium to the master disk as in the conventional technique. Accordingly, a magnetic pattern oriented oblique to the tracks can be formed well without a danger of damaging the medium or the mask, or without increasing defects in the medium.
Further, since the energy beams are used for heating a local portion of the magnetic recording medium, it is easy to control the size of the portion to be heated and power used whereby a magnetic pattern can be formed accurately.
Further, when the photomask is once prepared, a magnetic pattern having a desired shape can be formed in the medium. Accordingly, a complicated pattern or a unique pattern which was difficult to form in the conventional technique can easily be formed.
For example, in a phase servo system for a magnetic disk, a magnetic pattern which extends from an inner periphery to an outer periphery linearly in a oblique direction to the radius and the tracks, is used. In the conventional servo pattern forming method wherein servo signals were recorded for each track while the disk was rotated, it was difficult to form a pattern continuous to the radial direction or a pattern extending oblique to the radial direction. In the present invention, however, complicated calculation or a complicated device structure is unnecessary, and such magnetic pattern can be formed easily in a short time by irradiating energy beams at a time.
It is not always necessary for the photomask to cover the whole surface of the magnetic disk but it may have a size sufficient to include a repetition unit for forming the magnetic pattern. Since such photomask can be used by moving successively, the magnetic pattern can be formed a simple and economical way.
Further, when the beam diameter of energy beams is formed to be a large diameter or a longitudinally elongated elliptical shape, a plurality of tracks or a plurality of sectors of a magnetic pattern can be irradiated in a lump. Accordingly, writing efficiency can further be increased, and a problem of taking much time in writing servo signals with a future increase of the capacity can preferably be improved.
The photomask can be such one capable of forming a tint (an intensity distribution) of energy beams in the surface of the magnetic disk so as to correspond to a magnetic pattern to be formed. However, it is preferable to use a photomask having a transmitting portion for transmitting energy beams according to a pattern in view of easiness and cost.
In the present invention, the photomask having a reflectivity of energy beam of 30% or less at both the transmitting portion and the non-transmitting portion in at least its one surface of the photomask which faces the magnetic recording medium, is used.
In the conventional technique in a field of semiconductor, anti-reflection coating is applied to a light exposure surface (a surface to which light is exposed) of the photomask, i.e., the surface, on the opposite side of a semiconductor substrate, of the photomask, in order to decrease the reflectivity of the surface. If the reflectivity of the light exposure surface is high, exposure light is reflected to thereby decrease efficiency in using optical energy. The provision of the anti-reflection layer is to prevent the reflection and allows to use the optical energy efficiently.
On the other hand, the present invention is featurized by decreasing the reflectivity of the surface, facing the magnetic recording medium, of the photomask.
In the field of semiconductor, since the exposure of light was conducted to a photoresist of high reflectivity disposed on a semiconductor substrate through a mask, there was no problem of the interference of light between the mask and the substrate. On the other hand, since the magnetic recording medium subjected to the exposure has a metallic layer and a carbon layer on its surface, and the reflectivity is generally very high in comparison with the photoresist, there was such problem that light is reflected to interfere mutually between the mask and the medium, whereby an interference fringe is produced.
According to the present invention, the production of an interference fringe can be suppressed by using a photomask which suppresses substantially the reflection of energy beams on its surface facing the medium, whereby it is possible to form an accurate magnetic pattern having a small modulation. The reflectivity usable in the present invention is 30% at a maximum. More preferably, the reflectivity of the surface, facing the magnetic recording medium, of the photomask is 20% or less. Further, the reflectivity of the photomask to energy beams is preferably low as possible. However, it is generally 0.01% or more.
By the reason as described above, the present invention provides a remarkable effect in the formation of a magnetic pattern in a magnetic recording medium having a reflectivity of energy beam of 30% or more. However, if the reflectivity of energy beam is excessively high, the absorption of the energy beams is not sufficient. Accordingly, the reflectivity of energy beam of the magnetic recording medium is preferably 90% or less.
The photomask may be a mask having a transmitting portion and a non-transmitting portion formed so as to correspond to a predetermined magnetic pattern. Generally, predetermined transmitting portion and non-transmitting portion can be formed by forming a metallic layer of Cr or the like by sputtering on a transparent substrate such as quartz glass, optical glass, soda lime glass or the like; coating a photoresist thereon and etching the photoresist to remove locally the metallic layer. In this case, the portion having the Cr layer on the substrate constitutes the non-transmitting portion of energy beam, and the portion without having any layer on the substrate constitutes the transmitting portion.
However, Cr used generally for forming the non-transmitting portion has a very high reflectivity. Accordingly, in the present invention, it is desirable to cover an outermost layer of the surface, facing the magnetic recording medium, of non-transmitting portion with a layer having a low reflectivity. For this, it is preferable that the outermost layer of the surface, facing the magnetic recording medium, of the non-transmitting portion is a chromium oxide layer.
Further, it is preferable that the outermost layer of the photomask of the present invention is covered with a dielectric layer. Namely, the dielectric layer formed as the outermost layer of the surface, facing the magnetic recording medium, of the non-transmitting portion decreases further the reflectivity. It is also preferable that when the outermost layer of the surface, facing the magnetic recording medium, of the transmitting portion is a dielectric layer, the reflection of light at the substrate surface of the photomask can be presented.
It is also preferable that the reflectivity of the surface, on the opposite side of the magnetic recording medium (light exposure surface), of the photomask is 30% or less. With this, the reflection of energy beams at the light exposure surface can be prevented, and energy can be used effectively. Accordingly, the irradiation power of energy beams can be reduced, and the possibility of damaging the photomask or the magnetic recording medium can be reduced, and the durability can be improved. In particular, the durability of the photomask in repetitive use can be increased.
In the magnetic pattern forming method of the present invention, energy beams of high power are irradiated, so that the photomask may be damaged in repetitive use. It is therefore significant to improve the possible damage to the photomask.
When the outermost layer of the surface, on the opposite side of the magnetic recording medium, of the photomask is constituted by a dielectric layer, the reflection can preferably be decreased and energy can effectively be used.
In forming the dielectric layer for anti-reflection, a coating layer which can reduce substantially the reflection of light to a specified wavelength is called, in particular, a V coat layer. When laser is used as energy beams, it is preferable to form the V coat layer by taking advantage of mono-wavelength properties. When energy beams of an ultraviolet region (wavelength: 200-300 nm) are used for a quartz glass substrate, the reflectivity is about 5% of incident light. However, by forming the dielectric layer, the reflectivity can be reduced to 1% or less and a remarkable improvement of the modulation can preferably be obtained.
Although such dielectric layer can be formed by a sputtering method or a vapor deposition method, it is preferable to use the sputtering method in order to obtain a dielectric layer having a high durability to energy beams. In the layer forming technique, there is a danger that the dielectric layer is peeled off during the repetitive use of the photomask by the irradiation of pulsed energy beams such as pulsed laser of high power in order to form correctly a magnetic pattern because the pulsed laser has a high peak value of energy density. The power per pulse of the pulsed energy beams is preferably from 10 mJ/cm2 to 1,000 mJ/cm2.
In particular, it is preferable to use the sputtering method when the dielectric layer is formed on the surface having recesses and projections.
An anti-reflection layer made of a dielectric layer used in the conventional semiconductor field was generally formed on a flat surface without recesses and projections, such as a light exposure surface of the photomask. The surface, facing the magnetic recording medium, of the photomask has generally a structure having recesses and projections, which is resulted from forming a non-transmitting layer on a light transmitting substrate. In such surface having recesses and projections, a stress generates easily in a corner portion whereby the dielectric layer easily peels off from such surfaces in comparison with the flat surface.
Further, it is preferable that the substrate of the photomask is made of a material containing quartz as the main component. Namely, the quartz has a high permeability of energy beam in a ultraviolet region, and it is advantageous in using energy beams of a short wavelength of 300 nm or less which allows fine processing easily.
In the formation of the magnetic pattern of the present invention, it is preferable that the photomask is disposed with a space of 1 mm or less between the photomask and the magnetic recording medium. If the distance is larger than 1 mm, the diffraction of the energy beams is large, and the shape of the formed magnetic pattern is unclear.
According to the magnetic recording medium of the present invention in which a magnetic pattern is formed by using the photomask of the present invention and according to the method of the present invention, good results of less influence of the interference fringe; highly accurate magnetic pattern, and a modulation of the output signal of the magnetic pattern being 25% or less are obtainable. In particular, when a servo pattern is formed, a large effect is obtainable because the size of the modulation influences largely the precision of position determination. The modulation (Mod) at this moment is expressed by Mod=(AMPmaxxe2x88x92AMPmin)/TAAxc3x97100 where TAA (Total Average Amplitude) is an averaged output in the same pattern area, and AMPmax and AMPmin are respectively the maximum value and the minimum value in that area. The modulation value is an index of the uniformity of signals, and the smaller the value is, the better. In this case, TAA, AMPmax and AMPmin are all values in peak-to-peak. The value of the modulation is preferably 25% or less, more preferably, 10% or less in consideration of servo tracking accuracy.
The magnetic recording device of the present invention is characterized by comprising a magnetic recording medium, driving means for driving the magnetic recording medium in a recording direction, a magnetic head having a recording portion and a reproducing portion, means for moving relatively the magnetic head with respect to the magnetic recording medium, and recording/reproducing signal processing means which supplies a recording signal to the magnetic head and receives a reproducing signal from the magnetic head. As the magnetic recording medium, a magnetic recording medium in which a magnetic pattern such as a highly accurate servo pattern or the like is formed according to the present invention, is used, whereby high density recording is possible and error-free recording can be conducted because there is few flaw and defect in the medium.
By using such magnetic recording device in which the magnetic recording medium is assembled, it is possible to obtain signals by reproducing the magnetic pattern by the magnetic head and to record servo burst signals based on the signals by the magnetic head.